Carrier-phase differential GPS (CDGPS) is a promising technology for accurate relative navigation in LEO formations of\r\ncooperating satellites, but navigation filter robustness against poor GPS geometry and noisy measurements has to be improved.\r\nThis can be performed by augmenting the navigation filter with intersatellite local ranging measurements, as the ones provided by\r\nranging transponders or GNSS-like systems. In this paper, an augmented CDGPS navigation filter is proposed for the formation of\r\ntwo satellites characterized by a short, varying baseline, relevant to next generation Synthetic Aperture Radar missions. Specifically,\r\na cascade-combination of dynamic and kinematic filterswhich processes double-differenced code and carrier measurements on two\r\nfrequencies, as well as local inter-satellite ranging measurements, is used to get centimeter-level baseline estimates.The augmented\r\nfilter is validated by numerical simulations of the formation orbital path. Results demonstrate that the proposed approach is effective\r\nin preserving the centimeter-level accuracy achievable by a CDGPS-only filter also in the presence of a poor GDOP or a limited\r\nnumber of GPS satellites in view.
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